# Krueger flap

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Aerodynamic device

Krueger flaps deployed from the [leading edge](/source/Leading_edge) of a [Boeing 747](/source/Boeing_747) (top left and right in photo).

**Krueger flaps**, or **Krüger flaps**, are lift enhancement devices that may be fitted to the [leading edge](/source/Leading_edge) of an aircraft [wing](/source/Wing). Unlike [slats](/source/Leading-edge_slat) or [droop flaps](/source/Leading-edge_droop_flap), the main wing upper surface and its leading edge is not changed. Instead, a portion of the lower wing is rotated out in front of the main wing leading edge. The [Boeing 707](/source/Boeing_707) and [Boeing 747](/source/Boeing_747) used Krueger flaps on the wing leading edge. Several modern aircraft use Krueger flaps between the fuselage and closest engine, but use [slats](/source/Leading-edge_slat) outboard of the closest engine. The [Boeing 727](/source/Boeing_727) also used a mix of inboard Krueger flaps and outboard slats, although it had no engine between them.

## Operation

While the aerodynamic effect of Krueger flaps may be similar to that of [slats](/source/Leading_edge_slats) or [slots](/source/Leading_edge_slot) (in those cases where there is a gap or slot between the flap trailing edge and wing leading edge), they are deployed differently. Krueger flaps, hinged at their foremost position, hinge forwards from the under surface of the wing, increasing the [wing camber](/source/Camber_(aerodynamics)) and maximum [coefficient of lift](/source/Coefficient_of_lift).[1] It produces a nose-up [pitching moment](/source/Pitching_moment). Conversely, slats extend forwards from the upper surface of the leading edge. Also, when deployed, Krueger flaps result in a much more pronounced blunt leading edge on the wing, helping to achieve better low-speed handling. This allows smaller-radius wing leading edges, better optimized for cruise. Leading edge Krueger flaps enhance wing's low speed [lift](/source/Lift_(force)) production especially on [swept wing](/source/Swept_wing) aircraft. [2]

## Variable camber Krueger flap

The Krueger flaps developed for the [Boeing 747](/source/Boeing_747) were constructed from [fiberglass](/source/Fiberglass) material and were designed to be intentionally distorted into a much more efficient [aerofoil](/source/Aerofoil) section on deployment.[3]

Invented by James B. Cole and Richard H. Weiland of Boeing in the mid-1960s,[4] the "VCK" (Variable Camber Krueger) flaps deployed from the lower leading edge of the wing similar to rigid panel Krueger flaps. The high-speed lower wing in that region of the wing is a straight line normal to the wing leading edge, so the stowed panels are nominally flat, albeit twisted a small amount along the leading edge of the wing. Using two sets of identical linkages per flap, the fiberglass panel is deployed and bent to an optimal aerodynamic shape for low speed flight, while a separate aluminum folding nose that is stowed inside the wing is deployed tangent to the fiberglass panel.[5]

The [Boeing 747-8](/source/Boeing_747-8) wing was redesigned with optimized VCK flap panels that were similar to those on the original 747.

Another airplane that used VCK flaps was the [Boeing YC-14](/source/Boeing_YC-14).

## History

Krüger flaps were invented by [Werner Krüger](/source/Werner_Kr%C3%BCger) in 1943 and evaluated in the wind tunnels in Göttingen, Germany.[6] One of the earliest civil applications was the [Boeing 707](/source/Boeing_707), whereas the Swiss company [FFA](/source/Flug-_und_Fahrzeugwerke_Altenrhein) claimed the first use of the flap in its [FFA P-16](/source/FFA_P-16) fighter which flew in 1955.[7] The flap was added to prevent [wing stall](/source/Stall_(fluid_dynamics)) with an extreme attitude take-off with the tail dragging on the runway, a scenario that had caused two [de Havilland Comet](/source/De_Havilland_Comet) accidents. A preliminary flight test had been made on the [Boeing 367-80 (the Dash 80)](/source/Boeing_367-80) using a fixed flap and a skid on the after-body.[8] After the Boeing test flight on the B-707 prototype on 15 July 1954, Krueger flaps were first used in production for the Boeing 727 which made its maiden flight on 9 February 1963.[9]

		- Krueger flap operation

		- Slat operation

[Boeing](/source/Boeing) conducted a series of test flights in 2015 with a modified [Boeing 757](/source/Boeing_757), incorporating new wing-leading-edge sections and an [actively blown](/source/Active_flow_control) vertical tail.[10] The left wing was modified to include a 6.7 m-span glove section supporting a variable-camber Krueger flap to be deployed during landing, protruding just ahead of the leading edge. Although Krueger flaps had been tried before as insect-mitigation screens, previous designs caused additional drag. The newer design is variable-camber and retracts as seamlessly as possible into the lower wing surface. Increasing [natural laminar flow](/source/Natural_laminar_flow) (NLF) on an aircraft wing can reduce fuel burn by as much as 15%, but even small contaminants from insect remains could trip the flow from laminar to turbulent, destroying the performance benefit. The test flights were supported by the European airline group [TUI AG](/source/TUI_AG) and conducted jointly with [NASA](/source/NASA) as part of the agency’s Environmentally Responsible Aviation (ERA) program.

## See also

- [Index of aviation articles](/source/Index_of_aviation_articles)

- [Flap (aeronautics)](/source/Flap_(aeronautics))

- [High-lift device](/source/High-lift_device)

- [Leading-edge extension](/source/Leading-edge_extension)

## References

1. **[^](#cite_ref-1)** Gary V. Bristow (2002). [*Ace the Technical Pilot Interview*](https://books.google.com/books?id=aGL035btsg4C). McGraw-Hill Professional. [ISBN](/source/ISBN_(identifier)) [0-07-139609-8](https://en.wikipedia.org/wiki/Special:BookSources/0-07-139609-8). Retrieved 2009-02-16.

1. **[^](#cite_ref-Wyatt_2-0)** Wyatt, David (21 August 2014). [*Aircraft Flight Instruments and Guidance Systems: Principles, Operations and ...*](https://books.google.com/books?id=uxNUBAAAQBAJ&q=Werner+Kr%C3%BCger+%281910-2003%29&pg=PA126) Routledge. [ISBN](/source/ISBN_(identifier)) [9781317938316](https://en.wikipedia.org/wiki/Special:BookSources/9781317938316). Retrieved 16 July 2020.

1. **[^](#cite_ref-3)** Taylor 1990, p. 114.

1. **[^](#cite_ref-4)** U.S. Patent 3,504,870

1. **[^](#cite_ref-5)** J. B. Cole (Dec 17, 1966), Design of the Variable Camber Flap

1. **[^](#cite_ref-KlußmannMalik2012_6-0)** Niels Klußmann; Arnim Malik (2012). [*Lexikon Der Luftfahrt*](https://books.google.com/books?id=nUMYE0Viwg4C&pg=PA193). Springer. pp. 193–. [ISBN](/source/ISBN_(identifier)) [978-3-642-22500-0](https://en.wikipedia.org/wiki/Special:BookSources/978-3-642-22500-0).

1. **[^](#cite_ref-7)** X-Planes of Europe II, Tony Buttler Hikoki Puplication 2015. Page 193. [ISBN](/source/ISBN_(identifier)) [978-1-9021-0948-0](https://en.wikipedia.org/wiki/Special:BookSources/978-1-9021-0948-0)

1. **[^](#cite_ref-8)** "The Road to the 707" Cook, William H., TYC Publishing Company, Bellevue, 1991, [ISBN](/source/ISBN_(identifier)) [0-9629605-0-0](https://en.wikipedia.org/wiki/Special:BookSources/0-9629605-0-0), p.249

1. **[^](#cite_ref-Hitchens_9-0)** Hitchens, Frank (25 November 2015). [*The Encyclopedia of Aerodynamics*](https://books.google.com/books?id=Zjv0CgAAQBAJ&q=Werner+Kr%C3%BCger+%281910-2003%29&pg=PT372). Andrews UK Limited. [ISBN](/source/ISBN_(identifier)) [9781785383250](https://en.wikipedia.org/wiki/Special:BookSources/9781785383250). Retrieved 16 July 2020.

1. **[^](#cite_ref-10)** ["757 EcoDemo Focuses On Laminar And Active Flow"](http://aviationweek.com/technology/757-ecodemo-focuses-laminar-and-active-flow?NL=AW-19&Issue=AW-19_20150323_AW-19_254&sfvc4enews=42&cl=article_6). Aviation Week. 23 March 2015. Retrieved 23 March 2015.

## Sources

Wikimedia Commons has media related to [Krueger flaps](https://commons.wikimedia.org/wiki/Category:Krueger_flaps).

- Taylor, John W.R. *The Lore of Flight*, London: Universal Books Ltd., 1990. [ISBN](/source/ISBN_(identifier)) [0-9509620-1-5](https://en.wikipedia.org/wiki/Special:BookSources/0-9509620-1-5).

v t e Aircraft components and systems Airframe structure Aft pressure bulkhead Cabane strut Canopy Crack arrestor Cruciform tail Dope Empennage Fabric covering Fairing Flying wires Former Fuselage Hardpoint Interplane strut Jury strut Leading edge Lift strut Longeron Nacelle Rib Spar Stabilizer Stressed skin Strut T-tail Tailplane Trailing edge Triple tail Twin tail V-tail Vertical stabilizer Wing root Wing tip Wingbox Flight controls Aileron Airbrake Artificial feel Autopilot Canard Centre stick Deceleron Dive brake Dual control Electro-hydraulic actuator Elevator Elevon Flaperon Flight control modes Fly-by-wire Gust lock HOTAS Rudder Rudder pedals Servo tab Side-stick Spoiler Spoileron Stabilator Stick pusher Stick shaker Trim tab Wing warping Yaw damper Yoke Aerodynamic and high-lift devices Active Aeroelastic Wing Adaptive compliant wing Anti-shock body Blown flap Channel wing Dog-tooth Drag-reducing aerospike Flap Gouge flap Gurney flap Krueger flap Leading-edge cuff Leading-edge droop flap LEX Slats Slot Stall strips Strake Variable-sweep wing Vortex generator Vortilon Wing fence Winglet Avionic and flight instrument systems ACAS Air data boom Air data computer Aircraft periscope Airspeed indicator Altimeter Annunciator panel Astrodome Attitude indicator Compass Course deviation indicator EFIS EICAS Flight management system Glass cockpit GPS Head-up display Heading indicator Horizontal situation indicator INS ISIS Multi-function display Pitot–static system Radar altimeter TCAS Transponder Turn and slip indicator Variometer Yaw string Propulsion controls, devices and fuel systems Autothrottle Drop tank FADEC Fuel tank Gascolator Inlet cone Intake ramp NACA cowling NACA duct Self-sealing fuel tank Splitter plate Throttle Thrust lever Thrust reversal Townend ring War emergency power Wet wing Landing and arresting gear Aircraft tire Arrestor hook Autobrake Conventional landing gear Drogue parachute Landing gear Landing gear extender Oleo strut Tricycle landing gear Tundra tire Escape systems Ejection seat Escape crew capsule Other systems Aircraft lavatory Auxiliary power unit Bleed air system Deicing boot Emergency oxygen system Environmental control system Flight recorder Hydraulic system Ice protection system In-flight entertainment system Landing lights Navigation light Passenger service unit Ram air turbine

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Adapted from the Wikipedia article [Krueger flap](https://en.wikipedia.org/wiki/Krueger_flap) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Krueger_flap?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.